1. Field of the Invention
The present invention relates to a process of purifying polyhydroxycarboxylic acid which is used as a biodegradable polymer for medical plastics and as a substitute for general purpose resins and relates to polyhydroxycarboxylic acid which has an extremely low content of a catalyst.
2. Related Art of the Invention
Polyhydroxycarboxylic acid has high mechanical strengths and excellent physical and chemical properties, and is additionally biodegradable, that is, capable of being decomposed in the natural environment without giving hazardous effect and finally being converted to water and carbon dioxide by microorganisms. Consequently, polyhydroxycarboxylic acid has recently been used for medical plastics and as a substitute for general purpose resins, and has received attention in many fields of use. Particularly, environmental contamination is now an urgent problem to be solved and thus such plastic meets social requirements.
It has generally been known as a preparation process of polyhydroxycarboxylic acid that hydroxycarboxylic acid such as lactic acid and glycolic acid is dimerized by dehydration and the resulting cyclic dimer is subjected to a ring opening polymerization in the presence of a Sn-base catalyst to give polyhydroxycarboxylic acid. Such a process, however, is complex in the reaction procedures and the polymer obtained is expensive. Further, in the process, the polymer is prepared by melt-polymerization and pelletized as intact. As a result, the polymerization catalyst is unfavorably retained in the polymer and sometimes leads to give a very injurious effect on the uses of the final product. For example, in the case of slow-release medicines and other administrations to organisms, the polymer is decomposed whereas tile catalyst remains as intact in the tissue. Consequently, the polymer cannot be used because of toxicity of the catalyst. The decomposition rate of the polymer also differs depending upon the residual amount of the catalyst and the slow-release property of the medicine cannot be fully exhibited. As a result, uses of the polymer are greatly restricted by the catalyst and other impurities contained.
On the other hand, several processes which can prepare the polymer without using the above catalyst have been proposed. For example, European Patent 26599 and Japanese Laid-Open Patent SHO 56-45920 have disclosed a process for preparing a copolymer from glycolide and lactide by using a weakly acidic ion exchanger.
European Patent 171907 and Japanese Laid-Open Patent SHO 61-28521 have disclosed a process for preparing homopolymer or copolymer of lactic acid and glycolic acid by dehydrating polycondensation. It has also been disclosed a process for conducting polymerization in the presence of an inorganic solid acid catalyst. Any of these processes, however, provides a polymer having a low weight average molecular weight of 5,000.about.30,000. The polymer has low mechanical strengths and is unsatisfactory in chemical and physical properties, and thus has been remarkably restricted in the field of use.
As described above, in the preparation of polyhydroxycarboxylic acid having sufficiently high molecular weight, contamination with the catalyst is inevitable in the present technique unless purification is successively carried out.
Several purification processes have been disclosed in order to remove the catalyst from the polymer.
For example, Japanese Laid-Open Patent SHO 63-145327 has disclosed a process for dissolving a catalyst containing polymer in a water-immiscible organic solvent, bringing the resulting solution into contact with water or an aqueous layer containing an inorganic acid, water soluble organic acid or water soluble complexing agent, separating the organic layer and successively separating the polymer by a known process. Japanese Laid-Open Patent SHO 63-254128 has disclosed a process for carrying out purification by dissolving the polymer in a good solvent and adding a precipitant in a turbulent shear state. These processes can remove the catalyst from the polymer, but have the following problems in industry.
In the process of Japanese Laid-Open Patent SHO 63-145327, first, the polymer solution in the organic solvent has high viscosity and leads to poor contact efficiency with the aqueous layer such as inorganic acid under usual stirring. Thus, the efficiency for removing the catalyst is not so good. Second, the mixture of the polymer solution in the organic solvent with the aqueous layer is extremely difficult to separate from each other. As a result, polymer concentration must be reduced in order to improve separation and volume efficiency is remarkably impaired. Third, it is difficult to precipitate and isolate the polymer.
Japanese Laid-Open Patent SHO 63-254128 carries out precipitation and purification of the polymer at the same time. The process is more simple as compared with the above process and nevertheless, has a problem of requiring specific equipment.
In any of the above processes, steps of dissolving the polymer in a good solvent and successively adding a poor solvent to precipitate and isolate the polymer are required for removing the catalyst from the polymer. As a result, the kinds of the solvent used have been increased and a great deal of labor and equipment cost has been required. Thus, a satisfactory purification process has not yet been found.